channels: Remove 'signature' from <channel-introduction>.

channels: Remove 'signature' from <channel-introduction>.

In the end signing the commit/key pair does not buy us much.  Someone
publishing a valid but different commit/key pair would effectively be
publishing a different channel, which could be a fork (made by a former
authorized developer) or simply a mirror.  In the latter case, there's
nothing to be gained by publishing a different commit/key pair.

* guix/channels.scm (<channel-introduction>)[signature]: Remove.
(make-channel-introduction): Adjust accordingly.